Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 302

representative of the non-specialized trails and shallow burrows ichnocoenosis,
while Surculichnus bifurcauda and other arthropod trace fossils compose the
arthropod trackway-dominated ichnocoenosis (see Supplementary Table 1 in
http://booksite.elsevier.com/9780444538130 ).

3.3 Ichnocoenoses and Ichnofacies of
Glaciolacustrine Rhythmites
The general structure of the trace-fossil assemblage and the ichnotaxa recorded
in the varve-like rhythmite deposits formed by deglaciation during Gondwana
and Holarctic-Antarctic ice ages are consistent with those of freshwater ichno-
facies. The study developed by Uchman et al. (2008, 2009) in the Lithuanian
varves reveals that the lateral distribution of trace fossils is patchy and can
change from abundant to absent at distances of 15-20 m without any difference
in lithology ( Fig. 4 ). The denser patches may have developed around more
nutritionally dense locales ( Uchman et al., 2009 ). The tracemakers moved
intensively within a patch or rarely between the patches (cf. Koy and
Plotnick, 2007 ). Densely looping trails can point to activity within highly nutri-
tional patches, whereas less tightly looping to winding forms (transitions to
Helminthoidichnites ) can indicate activity within less nutritional areas or
searches for a new patch. A change in trace-fossil composition at adjacent levels
may reflect changes in the activity of particular tracemakers. Cochlichnus and
Glaciichnium commonly occur alone, but rarely together. Probably, trace-
makers of Glaciichnium ( Asellus or a similar isopod crustacean) and
Cochlichnus (dipteran larvae or nematodes) were the most successful in the
Quaternary, being opportunistic colonizers of new substrates under stressing
environmental conditions ( Uchman et al., 2008 ).
The lacustrine varve chronology of Pleistocene glacial Lake Hitchcock
(Connecticut Valley, USA) spans 4000 varve years (see Ridge, 2003, 2004

and references therein) andwas recently recalibrated (J. Ridge, personal commu-
nication, 2010). The vertical distribution of its trace-fossil content allowed
Benner et al. (2009) to reconstruct the biotic occupation history of the deep gla-
cial lakes. The initial occupation stage of the lake bottom is represented by a
Cochlichnus assemblage preserved in oldest varves, closest to receding ice
and nearest underlying till. Cochlichnus isp. ( Fig. 5 ) in two forms is present in
the first varves overlying till at all localities inwhich till can be located, indicating
the pioneering life habit of the causative organism(s). Stage 2 is identified by the
presence of the fish-produced trace fossil, Undichna ( Fig. 5 H), in addition to
Cochlichnus . Stage 2 assemblages persist for circa 1.7 ka, after which a newben-
thic fish-produced trace fossil, Broomichnium flirii ( Fig. 5 G; see Benner et al.,
2008 ), and a variety of arthropod-produced trace fossils ( Fig. 5 B-E) including
Surculichnus bifurcauda ( Fig. 5 A; see Knecht et al., 2009 ) are added, producing
a Stage 3 assemblage. The shift from a depauperate Stage 2 to a Stage 3 assem-
blage, rich in arthropod trace fossils and having a higher diversity of fish trace

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